Conventionally, an image forming apparatus is known such as a laser printer that uses a belt for conveying sheets, performing intermediate transferring, and the like. In general, this type of belt (in the form of a belt unit) can be detached from the main body of the apparatus in order to enable the replacement or repair of the belt, typically after a predetermined period of use.
Normally, the belt unit is provided with a tension imparting component comprising a coiled spring or the like. The tension imparting component imparts a tensile force to the belt by urging a tension roller in an outward direction. The tension roller supports the belt and can change position in an inward or outward direction.
In contrast thereto, Japanese Laid-Open Patent No. 2001-209294 discloses an apparatus in which a tension imparting component is provided on the main body side of the apparatus. This tension imparting component is configured so that a predetermined tensile force is imparted to the belt. This is as a result of the tension imparting component urging the tension roller outward when the belt unit is mounted in the apparatus main body. In this apparatus, the structure of the belt unit is simplified because the tension imparting component is not provided in the belt unit.
In the configuration such as that of the previously described Japanese Laid-Open Patent No. 2001-209294, the belt has conventionally been formed of a hard material such as polyamide-imide. Therefore, changes in the peripheral length of the belt were relatively small. As a result, it was sufficient for the tension roller to have a relatively small moveable range. The belt did not slacken significantly even when the belt unit was detached from the apparatus main body.
Recently however due to demands for greater durability, there is an increasing use of belts that are made of a material that is more flexible than the conventional belt. Although this type of belt provides greater durability than a conventional belt material, such as polyamide-imide, belt elongation and changes in the peripheral length of the belt due to thermal expansion have increased.
When the use of a belt made of this kind of flexible material was attempted in the apparatus disclosed in the previously described Japanese Laid-Open Patent No. 2001-209294, it was necessary to increase the moveable range of the tension roller in order to correspond to the changes in the peripheral length of the belt. As a result, there was a problem such that the slackening of the belt increased when the belt unit was detached from the apparatus main body. Therefore, the belt was liable to be displaced along a width direction (axial direction of the rollers) and become detached from the belt unit.